Probably true. My thinking was that maybe there is a really deep layer of coral and detritus on the submerged streets, so a high monument built to somewhat wit

Probably true. My thinking was that maybe there is a really deep layer of coral and detritus on the submerged streets, so a high monument built to somewhat withstand falling hundreds of feet in a building collapse would end up in a shallower layer and be found more easily. You'd probably want more than one strategy for sending a message to the future. The historic plans for nuclear waste repositories can be interesting in that regard.